Phytoalexin antimicrobial properties

In some plant species, the production of stilbene monomers and oligomers has been shown to be associated with a variety of stress factors [2,112,120,121,122,123,124]. The stilbene 1 is one of the most commonly encountered of these stress metabolites. While 1 has been shown to exhibit rather modest antimicrobial properties, some of its oligomeric derivatives have demonstrated more potent phytoalexin activity [2,8,125,126]. 

Certain phenolic compounds can be biosynthesized de novo after infection. These compounds, which do not exist before infection and which have antimicrobial properties, are called phytoalexins. They are produced by plants as defense mechanisms in response to microbial infection [64,93], but the accumulated compounds are often flavonoids or coumarins, although benzoic acid itself has been shown in apple after infection. 

We can distinguish between secondary metabolites that are already present prior to an attack or wounding, so-called constitutive compounds, and others that are induced by these processes and made de novo. Inducing agents, which have been termed elicitors by phytopathologists, can be cell wall fragments of microbes, the plant itself, or many other chemical constituents (4,17,22-24). The induced compounds are called phytoalexins, which is merely a functional term, since these compounds often do not differ in structure from constitutive natural products. In another way this term is misleading, since it implies that the induced compound is only active in plant-microbe interactions, whereas in reality it often has multiple functions that include antimicrobial and antiherbivoral properties (see below). 

Pterocarpans of legume plants with the properties of phytoalexins are glyceoUins in soyabeans Glycinemax), medicarpin and maack-iain in chickpeas Cicer arietinum), pisatin in pea seeds Pisum sativum) and phaseoHn in common beans Phaseolus vulgaris). Pterocarpans occur preferentially as 6"-malonylglucosids. In addition to antimicrobial effects they also exhibit other biological effects (see Section 10.3.3.5.1). 

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